Heterogeneous multi-layer MMIC assembly

ABSTRACT

An HPA MMIC assembly includes a MMIC device coupled to a thermal spreader. A ground plane is provided on the thermal spreader and coupled to FETs in the MMIC device. The multiple levels of metal separated by multiple dielectric layers provide low-loss broad-band microstrip circuits. The thermal spreader may include diamond, an air/wire-edm spreader or a multi-layer board (MLB) with heat sink vias and ground vias.

BACKGROUND

A Monolithic Microwave Integrated Circuit (MMIC) device operates atmicrowave frequencies in the range of 300 MHz to 300 GHz. MMIC devicesinclude microstrips, or microstrip circuits, to carry themicrowave-frequency signals. A microstrip is an electrical transmissionline that can be fabricated using printed circuit board technology andgenerally includes a conducting strip separated from a ground plane by adielectric layer or substrate. A microstrip is less expensive toimplement when compared with a waveguide, however, the microstripgenerally has lower power handling capacity and higher losses and,because a microstrip is not enclosed, it is susceptible to cross-talkand unintentional radiation. Accordingly, MMIC designers need to designlow-loss broad-band micro-strip circuits that will operate properly atthese frequencies.

There are many known approaches to implementing micro-strip circuits.Decimated ground structures may be used to implement low-loss broad-bandcircuits, however, these are not compatible with grounded environments.Periodically perforated ground structures, where ground is brought to atopside of the MMIC through vias, provide a slow wave structure but nota low-loss circuit.

What is needed, therefore, is a better structure for providing low-lossmicro-strip circuits.

SUMMARY

In one aspect of the present disclosure, there is a monolithic microwaveintegrated circuit (MMIC) assembly, comprising: a MMIC device having adevice patterned ground plane layer; a device patterned circuit layerdisposed on a first surface of the MMIC device; a thermal spreadercoupled to the MMIC device; and a spreader ground plane disposed on thethermal spreader, wherein the device ground plane layer and the spreaderground plane are coupled to one another, wherein the thermal spreadercomprises a dielectric material disposed therein, and wherein a portionof the device patterned circuit layer is adjacent a surface of thethermal spreader not covered by the spreader ground plane, whereby a lowground inductance is provided to the MMIC device.

In another aspect of the present disclosure, there is an integratedcircuit (IC) assembly, comprising: a heat dissipation structure; adissipation structure ground plane disposed on the dissipationstructure; and an IC device, coupled to the dissipation structure,having a device patterned ground plane layer and a device patternedcircuit layer disposed thereon, wherein the device ground plane layerand the dissipation structure ground plane are coupled to one another,wherein the thermal dissipation structure comprises a dielectricmaterial, and wherein a portion of the device patterned circuit layer isadjacent a surface of the thermal dissipation structure not covered bythe dissipation structure ground plane, whereby a low ground inductanceis provided to the IC device.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the disclosure are discussed below with reference tothe accompanying figures. It will be appreciated that for simplicity andclarity of illustration, elements shown in the drawings have notnecessarily been drawn accurately or to scale. For example, thedimensions of some of the elements may be exaggerated relative to otherelements for clarity or several physical components may be included inone functional block or element. Further, where considered appropriate,reference numerals may be repeated among the drawings to indicatecorresponding or analogous elements. For purposes of clarity, not everycomponent may be labeled in every drawing. The Figures are provided forthe purposes of illustration and explanation and are not intended as adefinition of the limits of the disclosure. In the Figures:

FIGS. 1A and 1B represent a MMIC assembly in accordance with an aspectof the present disclosure;

FIG. 2 is a perspective view of a metal spreader in accordance with anaspect of the present disclosure;

FIGS. 3A and 3B represent a MMIC assembly in accordance with an aspectof the present disclosure; and

FIGS. 4A and 4B represent a MMIC assembly in accordance with an aspectof the present disclosure.

DETAILED DESCRIPTION

Details are set forth in order to provide a thorough understanding ofthe aspects of the disclosure. It will be understood by those ofordinary skill in the art that these may be practiced without some ofthese specific details. In other instances, well-known methods,procedures, components and structures may not have been described indetail so as not to obscure the aspects of the disclosure.

MMICs that implement GaN-based transistors, and their related radiofrequency (RF) power amplifiers, are becoming more prevalent as thistechnology replaces traveling wave tubes in radar, electronic warfare(EW) systems and satellite communications. The GaN high electronmobility transistors (HEMTs) on silicon carbide, however, generate a lotof heat in very small areas typically measured in microns, i.e.,densities greater than 1 kW/cm². Micro-channel and micro-jet heat sinkscan dissipate high heat fluxes in high power electronic devices andmetallized chemical vapor deposition (CVD) diamond heat spreaders arealso often used to dissipate the concentrated heat flux.

Generally, and as will be described in more detail below, aspects of thepresent disclosure are directed to combining a thermal spreader inconjunction with a monolithic semiconductor circuit to provide multiplelevels of metal separated by multiple dielectric layers that enable theMMIC designer to create low-loss, broad-band microstrip circuits. Thethermal spreader may include patterned diamond, a metal spreader with anair/wire-edm cavity in one or more predetermined locations, or amulti-layer board (PCB) with heat sink/ground vias in one or morepredetermined areas under the MMIC.

Referring now to FIGS. 1A and 1B, in one aspect of the presentdisclosure, a multi-layer MMIC assembly 100 includes a MMIC 104interfacing with a metal spreader 108 having at least one cavity oropening 112 in the metal that is, in one non-limiting example, a depthD≈40 mils, and a dielectric value er≈1. The MMIC 104, in a non-limitingexample, may be a Raytheon RF Components (RRFC) MMIC with GaN/GaAstechnology, having a thickness T≈4 mils and a dielectric value er≈10. Afirst MMIC patterned ground plane 116 made from, for example, laserablated (or patterned) gold, is provided on a first surface 120, i.e., abottom surface, of the MMIC 104.

The metal spreader 108 also serves as the ground plane when coupled tothe ground plane 116. The metal spreader 108 and the first MMICpatterned ground plane 116 will “self-align” with one another whensolder provided between the two is wetted, i.e.,self-aligning/self-assembly occurs with a natural “etch stop” where goldis not present. If necessary, self-aligning structures may be placed oneither or both the MMIC 104 and the metal spreader 108 to ensure properalignment.

The FETs in the MMIC 104 are coupled to the metal spreader 108. One ormore vias 132 on the MMIC 104 bring ground to a topside ground plane 134on a top surface 136 and the bottom surface 120 of the MMIC 104 from themetal spreader 108. In addition, there is also provided a back sidemetal layer 140 on the first surface 120 that is not grounded, i.e., notconnected to the metal spreader 108. Advantageously, the non-groundedmetal layer 140 between the metal spreader 108 and the body of the MMIC104 allows for the implementation of new circuit topologies.

FIG. 2 is a perspective view of a metal spreader 200, having an aircavity 212 defined therein. A plurality of surface areas 220 areprovided to contact corresponding portions of the underside of the MMICdevice where the heat producing FETs are located.

Referring now to FIGS. 3A and 3B, in another aspect of the presentdisclosure, a multi-layer MMIC assembly 300 includes a MMIC 304interfacing with a diamond heat spreader 308 including CVD diamondmaterial 309 with a depth C≈40 mil and er≈5.6. The MMIC 304, in anon-limiting example, may be the RRFC MMIC referenced above. A firstMMIC patterned ground plane 316 made from, for example, laser ablated(or patterned) gold, is provided on a first surface 320, i.e., a bottomsurface, of the MMIC 304.

A first diamond spreader patterned ground plane 324, for example, madefrom laser ablated (or patterned) gold, corresponding to the first MMICpatterned ground plane 316 is provided around the diamond heat spreader308 and onto at least a portion of a first surface 338, i.e., a topsurface, of the diamond heat spreader 308. The patterned ground plane324 also serves as the ground plane when coupled to the ground plane316. The first spreader patterned ground plane 324 and the first MMICpatterned ground plane 316 will “self-align” with one another whensolder provided between the two is wetted, i.e.,self-aligning/self-assembly occurs with a natural “etch stop” where goldis not present. If necessary, self-aligning structures may be placed oneither or both the MMIC and spreader to ensure proper alignment.

The FETs in the MMIC 304 are coupled to the ground plane 324 on thediamond heat spreader 308. One or more vias 332 on the MMIC 304 bringground to a topside ground plane 334 on a top surface 336 and the bottomsurface 320 of the MMIC 304 from the ground plane 324 on the diamondheat spreader 308. In addition, there is also provided a back side metallayer 340 on the first surface 320 that is not grounded, i.e., notconnected to the diamond heat spreader 308. Advantageously, thenon-grounded metal layer 340 between the diamond heat spreader 308 andthe body of the MMIC 304 allows for the implementation of new circuittopologies. The metal layer 340 may or may not be in contact with themetal layer 324 on the surface 338 of the diamond spreader 308 inaccordance with desired circuit design parameters.

Referring now to FIGS. 4A and 4B, in another aspect of the presentdisclosure, a multi-layer MMIC assembly 400 includes a MMIC 404interfacing with a multi-layer board (MLB) 408 with heat sink vias andground vias with a depth P that varies with the number of layers. TheMMIC 404, in a non-limiting example, may be the RRFC MMIC referencedabove. A first MMIC patterned ground plane 416 made from, for example,laser ablated (or patterned) gold, is provided on a first surface 420,i.e., a bottom surface, of the MMIC 404.

A heat sink/ground via 424 in the MLB 408 made from, for example, plated(or patterned) gold, corresponding to the first MMIC patterned groundplane 416 is provided in specified locations around the MLB 408 and ontoat least a portion of a first surface 428, i.e., a top surface, of theMLB 408. The MLB via grounds 424 and the first MMIC patterned groundplane 416 will “self-align” with one another when solder providedbetween the two is wetted, i.e., self-aligning/self-assembly occurs witha natural “etch stop” where gold is not present. If necessary,self-aligning structures may be placed on either or both the MMIC 404and the MLB 408 to ensure proper alignment.

As an alternative to soldering the MMIC 404 to the MLB 404, acombination of conductive and non-conductive epoxy can be used. Theconductive epoxy can be applied in those regions of the MLB 408 wherethe thermal/ground vias are located and the non-conductive epoxy can beplaced in regions of the MLB 408 where there are no thermal/ground vias.

The FETs in the MMIC 404 are coupled to the MLB patterned ground 424 ofthe MLB 408. One or more vias 432 on the MMIC 404 bring ground to atopside ground plane 434 on a top surface 436 and the bottom surface 420of the MMIC 404 from the MLB spreader 408. In addition, there is alsoprovided a back side metal layer 440 on the first surface 420 that isnot grounded, i.e., not connected to the MLB 408. Advantageously, thenon-grounded metal layer 440 between the MLB 408 and the body of theMMIC 404 allows for the implementation of new circuit topologies.

The openings 438, i.e., portions where there is no gold deposited, inthe MLB via ground 424 of the MLB spreader 408 provide an MLB dielectric(er≈2) to ground.

It is to be understood that the disclosure is not limited in itsapplication to the details of construction and the arrangement of thecomponents set forth herein or illustrated in the drawings as it iscapable of implementations or of being practiced or carried out invarious ways. Also, it is to be understood that the phraseology andterminology employed herein are for the purpose of description only andshould not be regarded as limiting.

Certain features, which are, for clarity, described in the context ofseparate implementations, may also be provided in combination in asingle implementation. Conversely, various features, which are, forbrevity, described in the context of a single implementation, may alsobe provided separately or in any suitable sub-combination.

The present disclosure is illustratively described in reference to thedisclosed implementations. Various modifications and changes may be madeto the disclosed implementations by persons skilled in the art withoutdeparting from the scope of the present disclosure as defined in theappended claims.

What is claimed is:
 1. A monolithic microwave integrated circuit (MMIC)assembly, comprising: a MMIC device, having a device patterned groundplane layer, the MMIC device generating a heat profile across the devicepatterned ground plane layer during operation; a device patternedcircuit layer disposed on a first surface of the MMIC device; a thermalspreader coupled to the MMIC device; and a spreader ground planedisposed on the thermal spreader, the spreader ground plane having ashape corresponding to the heat profile of the MMIC device and whichcomprises a plurality of surface areas corresponding to heat producingcomponents of the MMIC device, wherein the device patterned ground planelayer and the spreader ground plane are coupled to one another, whereinthe thermal spreader comprises a dielectric material disposed therein,and wherein a portion of the device patterned circuit layer is adjacenta surface of the thermal spreader not covered by the spreader groundplane, whereby a low ground inductance is provided to the MMIC device.2. The assembly of claim 1, wherein each of the device ground planelayer and the spreader ground plane comprises a conductive material. 3.The assembly of claim 2, wherein the conductive material comprises gold.4. The assembly of claim 1, wherein: the thermal spreader comprises ametal with a cavity defined therein, and wherein air in the cavity isthe dielectric.
 5. The assembly of claim 4, wherein the portion of thedevice patterned circuit layer is adjacent the air cavity.
 6. Theassembly of claim 1, wherein: the thermal spreader comprises diamondmaterial, wherein the spreader ground plane is disposed around an outersurface of the thermal spreader, and wherein the diamond material is thedielectric.
 7. The assembly of claim 1, wherein: the thermal spreadercomprises a multi-layer board assembly, and wherein the spreader groundplane is disposed around an outer surface of the multi-layer boardassembly.
 8. The assembly of claim 7, wherein the portion of the devicepatterned circuit layer is adjacent the multi-layer board assembly. 9.The assembly of claim 7, wherein the multi-layer board assemblycomprises a plurality of board layers with heat sink vias and groundvias therebetween.